Before describing the present invention, it is useful to review some of the factors that determine the performance of a photosensor and prior art attempts to improve the performance of photosensors by reducing the effects of noise therein. A photosensor and its accompanying electronics will be referred to as a pixel.
The full well capacity of a photosensor refers to the total amount of charge that can be stored in the photosensor before overflowing into adjoining photosensors. Since this feature is a result of the photocharge being stored on a capacitor, which is in turn dependent on pixel area, the full well capacity of a photosensor is dependent on its physical size.
Dynamic range is defined as the maximum signal strength achievable by a photosensor divided by the noise in the photosensor. The maximum achievable signal strength by a photosensor is determined by the full well capacity of the photosensor. Furthermore, the noise in the photosensor is the sum of dark and read noise components. In other words, the dynamic range of a photosensor can be described by equation (1) below.
                              Dynamic          ⁢                                          ⁢          Range                =                              Full            ⁢                                                  ⁢            well            ⁢                                                  ⁢            capacity                                              Dark              ⁢                                                          ⁢              Noise                        +                          Read              ⁢                                                          ⁢              Noise                                                          (        1        )            
Looking at equation (1) it can be seen that one way of increasing the dynamic range of a photosensor is by increasing its full well capacity.
Responsivity is a measure of the effectiveness of a photosensor in converting incident electromagnetic radiation into electrical current or voltage, and is inversely related to the capacitance of the photosensor.
There are three main sources of noise that contribute to the degradation of the quality of an image: photon shot noise, dark noise and read noise.
Photon shot noise results from natural fluctuations in the number of photons detected by a photosensor and is caused by the quantum statistical nature of photon emission and detection. This randomness is manifested as a temporal and spatial fluctuation in the signal produced by the photosensor. Photon shot noise occurs even with an ideal noise-free light source and an ideal noise-free photosensor. Thus, photon shot noise imposes a fundamental limit on the responsivity of a photosensor insofar as it determines the minimum noise level achievable therein.
Photon shot noise is governed by Poisson statistics and is described by the square root of the number of photons hitting a photosensor per unit time (or in other words, the square root of the intensity or flux (F) of incident radiation). Therefore, increasing flux density (F) reduces the relative fraction of photon shot noise.
However, as will be recalled, the full well capacity of a photosensor provides an upper limit on the number of photons that can be integrated therein. Thus, any attempt to increase the flux density of the radiation incident on a photosensor and the detection thereof must be accompanied by an increase in the full well capacity of the photosensor.
Read noise is a combination of system noise components inherent in the conversion of photogenerated charge carriers into a measurable signal; processing of the resulting signal; and analog-to-digital (A/D) conversion thereof.
Thus, read noise determines the lower detection limit of a pixel. Unfortunately, one of the major components of read noise is pixel reset noise, which depends on the capacitance of the pixel and is given by
      Vnoise    =                  k        *                  t          c                      ,where k represents Boltzmann's constant (1.38 E-23 m2kgs−2K−1), T represents the operating temperature in degrees Kelvin and C represents the capacitance of the pixel. Accordingly, any attempt to reduce the read noise of a pixel (by reducing its full well capacity) conflicts with increasing the dynamic range and reducing photon shot noise.
Prior art photosensors will now be discussed. Photons impinging on a semiconductor are absorbed by its atoms and if they are of sufficient energy, the photons liberate electron(s) from the atoms. Should an electron be liberated in the presence of an electric field, the electron will be drawn away from its atom and will not recombine therewith. In a photosensor, this process is typically achieved using a reverse-biased P-N junction which forms a diode.
There are several ways of detecting the photogenerated charges in a photodiode. Referring to FIG. 1, the simplest method is to measure a voltage on a photodiode 2. The photodiode 2 is reset to a known voltage (Vrt2) before being exposed to photons. When exposed to photons, the charges produced in the photodiode 2 discharge the photodiode 2. After its exposure to photons, the voltage across the photodiode 2 is measured. This voltage will be referred to as the post-exposure voltage. Furthermore, the difference between the reset voltage (Vrt2) and the post-exposure voltage will be referred to as the voltage swing.
The voltage swing of a photodiode is proportional to the number of photoelectrons generated therein. The relationship between the voltage swing of a photodiode and the number of photoelectrons generated therein can be described by equation (2) below
                              Δ          ⁢                                          ⁢          V                =                  Q                      C            pd                                              (        2        )            where ΔV is the voltage swing, Q is the photogenerated charge in the photodiode and Cpd is the photodiode's intrinsic capacitance. This relationship will be referred to as the conversion gain of a photodiode.
The above technique works well for small pixels (e.g., <10 um) as the photodiode capacitance (Cpd) is small and hence the conversion gain of the photodiode is large. However, increasing pixel size does not increase voltage swing, since while more photons impinge on a larger photodiode, the voltage change produced by the resulting photocharge is negated by the increased photodiode capacitance. Consequently, there is in effect a reduced conversion gain from the larger pixel.
One approach to the problem of reducing read noise while increasing full well capacity is to employ a digital pixel as described in “Performance of a 4096 pixel photon counting chip”, Proc. SPIE Symposium on Optical Science, Engineering and Instrumentation, 19-24 Jul. 1998, San Diego, SPIE Proceedings, pp. 3445-31. This system effectively increases full-well capacity by adding bits in a counter and uses a charge amplifier to convert photogenerated charge into a voltage swing.
More particularly, referring to FIG. 2 the Medipix™ sensor 10 comprises a photodiode 12 connected to a charge integrator 14 that in turn comprises a feedback capacitor (Cfb) and a charge amplifier 15. The charge amplifier 15 includes two inputs: an inverting input Vinn and a reset input V0. The charge integrator 14 is connected to a monostable oscillator 16 and a comparator 18. The comparator 18 is in turn connected to a counter 20.
Referring to FIG. 3, the imaging process performed by the Medipix™ sensor comprises two phases: an illumination phase (P0) and a readout phase (P1). During the illumination phase (P0), the sensor is illuminated by incident radiation and photocharges are generated thereby in the photodiode. During the readout phase (P1), the sensor is no longer illuminated and the signals generated in the sensor are readout therefrom.
Referring to FIG. 2 in combination with FIG. 3, at the start of an illumination phase (P0) and before radiation has been emitted from a radiation source (not shown), a Start Frame (SF) signal is transmitted which resets the counter 20 and the charge integrator 14 (to reset voltage V0).
When radiation is emitted from the radiation source (not shown) and the Medipix™ sensor 10 is illuminated thereby, the charge integrator 14 detects the photogenerated charge produced in the photodiode 12 and the feedback capacitor Cfb produces a ramp signal Vout. The slope of which is proportional to the photocurrent, and inversely proportional to the feedback capacitance as shown in equation (3) below.
                                          ⅆ                          V              out                                            ⅆ            t                          =                              I            photo                                C            fb                                              (        3        )            
When the output from the charge integrator 14 reaches a threshold Vref (set externally to the sensor 10), the output (a COMP signal) from the comparator 18 is switched to a high state.
The COMP signal has two functions. In particular, the COMP signal is used to reset the charge integrator 14 (to reset voltage V0); and increment the counter 20.
Thus, each increment of the counter 20 corresponds to a photogenerated voltage of
      (                  V        ref            -              V        0              )    ×            C      fb        e  (where e=1.6×10−19 C). In other words, if the output from the counter 20 equals 42, a voltage (Vpd) of
  42  ×      (                  V        ref            -              V        0              )    ×            C      fb        e  has been produced by the photodiode 12.
A monostable oscillator 16 is used to ensure that a reset pulse (not shown) is long enough to completely discharge the integrator 14. Optionally, a switch (not shown) may be provided between the photodiode 12 and the charge integrator 14. This switch is opened during the reset of the charge integrator 14 and prevents any disturbance to the charge on the photodiode 12 during the resetting operation.
There are various methods of accessing the data in the counter 20. The simplest method is shown in FIG. 2, namely parallel output. For example, let there be a ten bit parallel bus 22 to which all the pixels in a given column are connected. When the sensor 10 wishes to access a row, it asserts a Pixel Read (PR) signal (which is common to all of the pixels in a row) which causes the counter 20 on this row to output its data. The Pixel Read (PR) signal also causes all of the counters on the other rows to switch into a high-impedance/tri-state mode.
An alternative data access method is a bit serial approach, as shown in FIG. 4. In this method, there is a single output from each pixel Pixi (and there is no access signal) wherein the data is daisy-chained (i.e., the output from one pixel Pixi is passed to the input of the next pixel Pixi+1). The advantage of this technique is that the number of conductors required in the Medipix™ sensor 10 is greatly reduced. Consequently, a greater proportion of any incident radiation can reach the photosensitive area of a pixel and the sensitivity of the pixel is improved.
A hybrid of the two data access methods is also possible (e.g., 5 bit parallel daisy-chain). This reduces the number of connectors required in the Medipix™ sensor and also halves the pixel rate.
Problems with the prior art will now be discussed. Referring to FIG. 2 of the Medipix™ sensor 10 a practical capacitance for the feedback capacitor Cfb is 5 fF. Furthermore, a voltage swing of less than 300 mV is impractical. Since charge equals voltage multiplied by capacitance, using the above values, 1.5 fC (which corresponds to 10,000 electrons) are required to increment the counter 20 by one count.
Accordingly, the problem with the Medipix™ approach is that a relatively large number of photogenerated electrons are required to increment the counter 20.
In the case of medical imaging devices, the Medipix™ sensor 10 is best suited for use in high dosage radiation systems. However, in many cases it is desirable to repeatedly expose a patient to low dosage radiation (e.g., multiple x-rays etc.). But because of the relatively large number (around 10 k) of photogenerated electrons required to increment the counter 20 in the Medipix™ sensor 10, the Medipix™ sensor 10 may have limited utility in these situations.
The above-mentioned parallel output approach requires as many conductors as the number of bits in the counter 20. While this approach produces a faster response, the large number of conductors required therein obscures part of the photosensors. Consequently, the photosensors' sensitivity is degraded with the parallel output approach. In a similar fashion, the bit serial output approach has two serious disadvantages: it is not possible to read out a sub-section array (i.e., no random access); and the readout time of the array is increased as more clock cycles are required to read the data from a photosensor.